1. Field of the Invention
The present invention relates to a semiconductor optical modulator and a method to manufacture the semiconductor optical modulator.
2. Related Background Arts
The U.S. Pat. No. 6,667,529, has disclosed a semiconductor optical modulator and a method to manufacture the same. The optical modulator, which is made of semiconductor materials able to grow epitaxially on an InP substrate, includes an anode made of a metal stack of titanium (Ti) and gold (Au). The Ti in the anode is in contact with the contact layer made of InGaAsP. A Japanese Patent Application published as JP-2009-244648A has disclosed another semiconductor optical modulator. The modulator disclosed therein includes a waveguide layer made of InGaAsP, an n-type InP buffer layer beneath the waveguide layer, an insulating InP layer covering the top and sides of the waveguide layer, and an n-type InP layer on the insulating InP layer. One of paired electrodes to apply an electric field in the waveguide layer is in contact with the InP buffer layer, while the other electride is on the n-type InP layer.
FIG. 9 is a perspective view of a conventional semiconductor optical modulator 100A that includes (1) a mesa 132 formed by etching a semiconductor layers on an n-type InP substrate 112, an n-type InP buffer layer 114, a core layer 116 of a multiple quantum well (MQW) type structure, a p-type upper cladding layer 118 made of InP, and a p-type contact layer 119 made of InGaAsP or InGaAs; (2) a resin layer 120 covering the sides of the mesa 132, and (3) the anode metal 126 including a first metal 126a, a second metal 126b, and a pad 126c. The first metal 126a, which is in contact with the contact layer 119 in the top of the mesa 132, may be a double layer of eutectic metal AuZn and gold Au to secure the ohmic contact with the contact layer 119, while, the second metal 126b may be a double layer of titanium (Ti) and gold (Au). The eutectic metal AuZn extends on the top of the resin layer 120 through an insulating film 124, which is generally made of an inorganic material, for instance, SiO2, SiN, or SiON. However, because the eutectic metal of AuZn shows inferior adhesion against the inorganic material, the metal 126a is easily peeled off from the insulating film 124.
One solution avoid peel-off of the metal 126a from the insulating film 124 is that the eutectic metal 126a is restrictedly formed within the opening 120c formed in the resin layer 120 in the top of the mesa 132 not to extend to the top 120a of the resin layer 120. However, it is strictly hard to execute the photolithography process within the restricted region of the opening 120c on the top of the mesa 132, because the resin layer 120 has a thickness greater than 2 μm or more because of the reduction of the parasitic capacitance between the anode electrode 126 and the semiconductor layers, 112 to 119.